Hi Bret,
If my recollection is correct, the majority of recycled nutrients are
returned from decomposition of the foliage and sapwood of the tree. Few
nutrients are rendered from decomposition of the heartwood but the value
of added humus from heartwood decomposition should not be
underemphasized.
I have always made it a personal rule to leave all material under three
inches dia. in the woods to return to the ground. Some areas, I think
in Europe, have laws requiring this "leave" as well as the bark from
marketed logs.
Maybe someone else can add to this.